Chapter 8: The Physical Layer
The Physical layer controls how data is placed on the communication media.
The role of the OSI Physical layer is to encode the binary digits that represent Data Link layer frames into signals and to transmit and receive these signals across the physical media - copper wires, optical fiber, and wireless - that connect network devices.
The delivery of frames across the local media requires the following Physical layer elements:
The technologies defined by these organizations include four areas of the Physical layer standards:
The three fundamental functions of the Physical layer are:
Encoding is a method of converting a stream of data bits into a predefined "code.
The method of representing the bits is called the signaling method. The Physical layer standards must define what type of signal represents a "1" and a "0".
Signaling Methods
Bits are represented on the medium by changing one or more of the following characteristics of a signal:
NRZ Signaling
Manchester Encoding
Although Manchester Encoding is not efficient enough to be used at higher signaling speeds, it is the signaling method employed by 10BaseT Ethernet (Ethernet running at 10 Megabits per second).
Code Groups
Advantages using code groups include:
The process of balancing the number of 1s and 0s transmitted is called DC balancing. This prevents excessive amounts of energy from being injected into the media during transmission, thereby reducing the interference radiated from the media.
The code groups have three types of symbols:
4B/5B
In this technique, 4 bits of data are turned into 5-bit code symbols for transmission over the media system. In 4B/5B, each byte to be transmitted is broken into four-bit pieces or nibbles and encoded as five-bit values known as symbols.
Bandwidth
The capacity of a medium to carry data is described as the raw data bandwidth of the media. Digital bandwidth measures the amount of information that can flow from one place to another in a given amount of time. Bandwidth is typically measured in kilobits per second (kbps) or megabits per second (Mbps).
Throughput
Throughput is the measure of the transfer of bits across the media over a given period of time. Due to a number of factors, throughput usually does not match the specified bandwidth in Physical layer implementations such as Ethernet.
In an internetwork or network with multiple segments, throughput cannot be faster than the slowest link of the path from source to destination. Even if all or most of the segments have high bandwidth, it will only take one segment in the path with low throughput to create a bottleneck to the throughput of the entire network.
Goodput is the measure of usable data transferred over a given period of time, and is therefore the measure that is of most interest to network users.
As shown in the figure, goodput measures the effective transfer of user data between Application layer entities, such as between a source web server process and a destination web browser device.
Unlike throughput, which measures the transfer of bits and not the transfer of usable data, goodput accounts for bits devoted to protocol overhead. Goodput is throughput minus traffic overhead for establishing sessions, acknowledgements, and encapsulation.
As an example, consider two hosts on a LAN transferring a file. The bandwidth of the LAN is 100 Mbps. Due to the sharing and media overhead the through put between the computers is only 60 Mbps. With the overhead of the encapsulation process of the TCP/IP stack, the actual rate of the data received by the destination computer, goodput, is only 40Mbps.
The timing and voltage values of these signals are susceptible to interference or "noise" from outside the communications system. These unwanted signals can distort and corrupt the data signals being carried by copper media. Radio waves and electromagnetic devices such as fluorescent lights, electric motors, and other devices are potential sources of noise.
Cable types with shielding or twisting of the pairs of wires are designed to minimize signal degradation due to electronic noise.
This cancellation effect also helps avoid interference from internal sources called crosstalk. Additionally, the different pairs of wires that are twisted in the cable use a different number of twists per meter to help protect the cable from crosstalk between pairs.
Single-mode and Multimode Fiber
Fiber optic cables can be broadly classified into two types: single-mode and multimode.
Single-mode optical fiber carries a single ray of light, usually emitted from a laser. Because the laser light is uni-directional and travels down the center of the fiber, this type of fiber can transmit optical pulses for very long distances.
Multimode fiber typically uses LED emitters that do not create a single coherent light wave. Instead, light from an LED enters the multimode fiber at different angles. Because light entering the fiber at different angles takes different amounts of time to travel down the fiber, long fiber runs may result in the pulses becoming blurred on reception at the receiving end. This effect, known as modal dispersion, limits the length of multimode fiber segments.
Wireless Standards:
IEEE 802.11a - Operates in the 5 GHz frequency band and offers speeds of up to 54 Mbps. Because this standard operates at higher frequencies, it has a smaller coverage area and is less effective at penetrating building structures. Devices operating under this standard are not interoperable with the 802.11b and 802.11g standards described below.
IEEE 802.11b - Operates in the 2.4 GHz frequency band and offers speeds of up to 11 Mbps. Devices implementing this standard have a longer range and are better able to penetrate building structures than devices based on 802.11a.
IEEE 802.11g - Operates in the 2.4 GHz frequency band and offers speeds of up to 54 Mbps. Devices implementing this standard therefore operate at the same radio frequency and range as 802.11b but with the bandwidth of 802.11a.
IEEE 802.11n
The IEEE 802.11n standard is currently in draft form. The proposed standard defines frequency of 2.4 Ghz or 5 GHz. The typical expected data rates are 100 Mbps to 210 Mbps with a distance range of up to 70 meters.
Three common types of fiber-optic termination and splicing errors are: